<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div>
        <p>
            alert(typeof 12345); //输出number<br>
            alert(typeof "abc"); //输出string<br>
            alert(typeof document); //输出object<br>
        </p>
    </div>
</body>

</html>

<script type="text/javascript">
    window.alert(typeof 123 + '///////123' + typeof 'abc' + '--abc' + typeof document + '--document')
    // alert(); //输出number
    // alert(); //输出string
    // alert(typeof document); //输出object
</script>